The invention relates generally to wind turbines, and, in particular, to a system and method for controlling power output of a wind turbine generator during a wind gust, based upon upwind wind information, while reducing tower fore-aft and side-to-side moments.
Wind turbines are regarded as environmentally friendly and relatively inexpensive alternative sources of energy. A wind turbine generator generally includes a wind rotor having a plurality of blades that transform wind energy into rotational motion of a drive shaft, which in turn is utilized to drive a rotor of an electrical generator to produce electrical power. In modem wind power generation systems, power output from a plurality wind turbine generators, comprising a “wind farm”, is typically combined for transmission to the grid.
Power output of a wind turbine generator generally increases with wind speed until a rated power output is reached. Thereafter, the power output is usually maintained constant at the rated value even with an increase in wind speed. This is generally achieved by regulating the pitching action of the blades in response to an increase in wind speed. With increase in wind speed beyond the rated power output, the blades generally are pitched toward feather (i.e., twisted to be more closely aligned with the direction of the wind), thereby controlling the angular speed of the rotor. As a result, generator speed, and consequently, generator output may be maintained relatively constant with increasing wind velocities.
In case of sudden gusts, wind speed may increase drastically in a relatively small interval of time. Maintaining the power output of the wind turbine generator constant during such sudden gusts calls for relatively rapid changes of the pitch angle of the blades. However, there is typically a time lag between the occurrence of a gust and the actual pitching of the blades based upon dynamics of the pitch control actuator and the inertia of the mechanical components. As a result, generator speed, and hence power, may increase considerably during such gusts, and may exceed the maximum prescribed power output level (also known as overspeed limit) causing the generator to trip, and in certain cases, the wind turbine to shut down. The overspeed limit is generally a protective function for the particular wind turbine generator and is based upon fatigue considerations of the mechanical components, such as the tower, drive train, and so forth. Moreover, sudden gusts may also significantly increase tower fore-aft and side-to-side bending moments due to increase in the effect of wind shear.
Accordingly, there exists a need for an improved mechanism to control pitching of the blades of a wind turbine to maintain power output of the generator during heavy gusts, while reducing tower fore-aft and side-to-side moments.